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How A-S/Trane do it

I am a homeowner who has an American Standard UD080R9V4K and its manual is revealing. At first thought, one would think the VS blower would ramp CFM up to high and down to low, and all points in between. Looking at the fan tables in the "Service Facts" manual page 13, it is not quite that way. Instead, there is a handful of discrete settings. One major benefit seems to be nearly constant CFM within a wide range of ESP conditions, compared to the other air handlers which vary CFM more depending on ESP.

According to the table, a 3.5 ton AC could be set to deliver 1400 CFM, and it would remain in the range of 1380-1402 CFM with ESP as low as 0.1 inch w.c., and as high as 0.9.This is at 400 CFM/ton, 350 and 450 are selectable with DIP switches. You have surely seen more variety of equipment, but I have not seen such a range of ESP rated acceptable.

The two-stage furnace runs at 860 CFM low, and 1243 CFM on high heating, at the "Medium" fan speed. One can select "Low" and "High", each setting is relatively flat no matter what the ESP.

Other parts of the manual identify different speeds, but I do believe each is a fixed percentage of the main one. The famous "Comfort-R" feature provides for 50% and 80% of the main speed. A footnote tells us "Continuous Fan Setting: Heating or Cooling airflow is approximately 50% of selected Cooling value", and another says "For Variable Speed: low speed airflows are approximately 30% of listed values". I am not sure, but think that last item refers to their two-stage air conditioning.

That's about it. You have a wide number of discrete choices which can be applied in your situation. Certainly the choices far outweigh the low-med-hi of the builder grade Consolidated furnace it replaced, and certainly it can keep up with 0.9 inch ESP within its specs. At one time I had downloaded manuals for several Trane models and I do not remember those capabilities being with the lower grade air handlers.

Perhaps this amateur's view of things will inspire a pro to offer a fuller explanation. Or more concise.